Chemotherapy drugs are destructive to cancer cells (and others, unfortunately)
Tamoxifen, while it is a cancer therapy, is not chemotherapy.
It's a common misconception that all drugs used in cancer treatment are chemotherapy. Tamoxifen is hormone therapy.
There are a number of different drugs which are commonly used to reduce the chances of recurrence or metastatic disease which work by either reducing the amount of hormones you produce or inhibiting your cancers access to them. That is a gross simplification.
Don't get too worried about Tamoxifen. Yes, it has side effects, but they vary wildly. It does give you a full on bitch slap when it comes to menopause, but I figure if we all live long enough we will have to deal with that sooner or later anyway.
Statistically, chemo may increase survival rates by 5-10% Once the chemo blitzes your cancer cells (hopefully) hormone therapies can add 10-20% to your survival rate.
